Generally, the process of preparing a flexographic printing plate from a photopolymer element includes the steps of "backflash", main image exposure, development or "washout", drying detackification, and post exposure.
The "backflash" exposure may be used with elements having a transparent support. Backflash generally uses a radiation source emitting a principal wavelength around 360 nm. It serves to sensitize the plate and establishes the depth of the plate relief. The backflash exposure gives the photopolymer film a uniform and relatively short exposure through the support, thereby photocrosslinking binder and monomer in the floor region.
The primary purpose of the "backflash", the main imagewise exposure and overall post-exposing is to effect polymerization. Actinic radiation from a variety of sources can be used. These include commercial ultraviolet fluorescent tubes, medium, high, and low pressure mercury vapor lamps, argon glow lamps, electronic flash units, photographic flood lamps, pulsed xenon lamps, carbon arc lamps, etc. The radiation source must emit an effective amount of radiation having a wavelength in the range of 230 to 450 nm, preferably 300 to 420 nm and, more preferably, 340 to 400 nm. For efficient photopolymerization, the wavelength is matched to the absorption characteristic of the photoinitiator present in the photopolymerizable layer. Exposure times vary from a few seconds to a few minutes, depending upon the output of the lamps, distance from the lamps, relief depth desired, and the thickness of the plate.
It has been mentioned in DE-OS 31 10 512 that metal vapor arc lamps can be kept in a state of readiness by heating the small bulb to the temperature needed for the required operating vapor pressure before turning the lamp on. Maximum light output is achieved immediately when the lamp is turned on. There is no teaching of obtaining a uniform distribution of light in an exposure apparatus having at least one fluorescent light tube.
The Asahi Company's AFP 2500 is an apparatus used to expose the photopolymerizable layer. This apparatus contains several fluorescent lights positioned in a parallel fashion with respect to each other. The lights are located beneath a support made of a material which is transparent to the radiation. The fluorescent lights are low-pressure mercury lamps in which the internal luminescent layer converts the radiation from the mercury vapor column into UV and/or visible light
One of the problems plaguing these commercial exposure apparatuses is that they are incapable of consistently providing a uniform distribution of light to large, e.g. 100 cm.times.150 cm, photopolymerizable plates having increased photosensitivity and, thus, requiring a shorter exposure time.